Polycrystalline silicon substrates are a type of a semiconductor substrate that have been used for forming solar cell elements. Polycrystalline silicon substrates are obtained by slicing a silicon ingot. The silicon ingot may be produced by, for example, a method of feeding melted silicon into a mold and solidifying the silicon. The silicon ingot may also be produced by a method of melting a silicon material in a mold and solidifying the resultant silicon melt in the mold. Silicon generally melts at a temperature of about 1414° C.
When a silicon ingot is produced by the methods described above, a mold is generally used that has a releasing layer composed of a carbon material or a silica material on the inner surface of the base of the mold. Typically, the releasing layer is composed of a powder of silicon nitride (Si3N4), silicon carbide (SiC), silicon dioxide (SiO2), or the like.
In the past a technique for forming the releasing layer has been used, in which these powders are mixed with a solution composed of a certain binder and a solvent to form a slurry. The inner surface of a mold base is then coated with the slurry by spraying to form a releasing layer over the inner surface of the mold base. A dispersing apparatus has also been used for mixing and dispersing solid particles by the rotor-stator principle in a fluid being dispersed.
A releasing layer formed from the silicon nitride powder has a weak adhesion to a mold base and the strength of the releasing layer is low. For this reason, portions of the releasing layer can flake off from the mold base and, as a result, a silicon melt can come into contact with and fuse with the inner surface of the mold base. When a silicon melt fused with the mold in this way is solidified and cooled in the mold, the difference in thermal shrinkage between the material of the mold base and silicon causes flaws in the silicon ingot. Examples of flaws are cracking of the silicon ingot, incapability of releasing the silicon ingot from the mold (i.e., the molded silicon ingot gets stuck in the mold), and chipping of the silicon ingot occurring during releasing of the silicon ingot from the mold.
Furthermore, when matter flaked off from a releasing layer enters a silicon ingot, a silicon substrate (wafer) obtained by slicing the ingot can contain the flaked matter as foreign matter. This can increase the proportion of rejected silicon substrates, and can cause breakage of a wire of a wire saw during slicing of a silicon ingot with the wire saw.
To overcome such problems, it has been proposed that a releasing layer be formed with a slurry containing silicon nitride powder in which each powder particle has an oxide layer with a thickness of 1 nm or more over the surface of the particle. Such oxide layers are formed by subjecting silicon nitride powder to an oxidation modification treatment. A releasing layer formed by this method can have an enhanced strength because the oxide layers on the surfaces of silicon nitride particles are softened and the silicon nitride particles can be bonded to one another. When bulky particles or clusters of particles such as cohered particles or fused particles are present in a releasing layer and are too large, flaking of the releasing layer may not be sufficiently suppressed. For this reason, the size of bulky particles or particle clusters may need to be reduced by controlling the thickness of the oxide layers of silicon nitride particles or the average particle size, including the thickness of an oxide layer, of silicon nitride particles.
Although use of the above method can enhance the strength of a releasing layer, generation of bulky particles or particle clusters may not be completely suppressed. The bulky particles or particle clusters form because oxide layers on the surfaces of silicon nitride particles are bonded or fused to one another in the standby period prior to the slurry coating of a mold base. Thus, the possibility that bulky particles or particle clusters flake and enter a silicon melt continues to exist when manufacturing a silicon ingot. When a mold base is coated with a slurry, cohered particles may exist on the surface of the resultant releasing layer, and hence, may prevent formation of a releasing layer with high flatness. When such bulky particles or particle clusters of a releasing layer flake and enter a silicon melt, most of the particles are melted in the silicon melt, but some particles may not be melted and may remain as small particles (fine particles) in the resultant silicon ingot. When a solar cell element is manufactured from a silicon substrate obtained by slicing a silicon ingot containing such fine particles, the fine particles may cause a leak current, which degrades the properties of the solar cell element. In addition to this, since the solar cell element is defective due to leakage, the process yield is decreased. Accordingly, there is a need for methods of manufacturing solar cell elements that reduce the presence of releasing layer flakes and particles.